[amelius]

I suppose the reason is that sometimes you want to parse more than just the number. For example, numbers separated by commas. In that case you will have to call the function repeatedly from your own parsing routine and advance the pointer.

[gpderetta]

Yes, if you are parsing in-place numbers in an existing string you do not necessarily know a-priori where the number end. You could have extra logic to look ahead to the end of the number before passing it from_chars, but a) it would require an additional pass and b) you could end up replicating part of the implementation of from chars.

from_chars is supposed to be a low lever routine, it must be possible to use it correctly, but it is not necessarily the most ergonomic.